﻿THE 
  ECHINOTHUEID^: 
  AND 
  THE 
  PERISCJROECHINID^E. 
  311 
  

  

  Genus 
  Arch^ocidaeis, 
  M'Coy, 
  1844*. 
  PL 
  XXIV. 
  fig. 
  1. 
  

   (Echinocrinus, 
  Agassiz, 
  1841. 
  Palceocidaris, 
  Desor, 
  1846.) 
  

  

  In 
  this 
  genus 
  the 
  ambulacra 
  are 
  narrow, 
  and 
  composed 
  of 
  two 
  rows 
  

   of 
  small 
  plates, 
  of 
  irregular 
  form, 
  each 
  provided 
  with 
  two 
  pores. 
  The 
  

   interambulacra 
  are 
  continuous 
  from 
  pole 
  to 
  pole, 
  and 
  are 
  composed 
  

   of 
  three 
  or 
  more 
  rows 
  of 
  plates, 
  each 
  provided 
  with 
  a 
  primary 
  tuber- 
  

   cle. 
  The 
  rows 
  of 
  plates 
  bordering 
  the 
  ambulacra 
  are 
  pentagonal 
  in 
  

   form, 
  the 
  middle 
  row 
  or 
  rows 
  hexagonal 
  ; 
  the 
  large 
  oblong 
  penta- 
  

   gonal 
  plates 
  have 
  two 
  of 
  the 
  sides 
  bevelled 
  from 
  the 
  upper 
  surface, 
  

   the 
  opposite 
  side 
  being 
  bevelled 
  on 
  the 
  lower 
  edge 
  ; 
  one 
  of 
  the 
  other 
  

   two 
  sides 
  has 
  a 
  small 
  groove 
  into 
  which 
  the 
  edge 
  of 
  the 
  next 
  plate 
  

   was 
  received 
  f; 
  the 
  -primary 
  tubercles 
  are 
  perforated 
  and 
  hollow 
  ; 
  

   apical 
  disk 
  unknown 
  ; 
  mouth 
  surrounded 
  apparently 
  by 
  a 
  membrane 
  

   bearing 
  numerous 
  minute 
  imbricated 
  plates 
  J. 
  

  

  Taking 
  this 
  genus 
  as 
  the 
  type 
  of 
  the 
  Perischoechinidce, 
  we 
  have 
  the 
  

   first 
  approach 
  towards 
  the 
  structure 
  of 
  the 
  Echinothuridce, 
  as 
  typified 
  

   by 
  Calveria. 
  Whether 
  the 
  interambulacral 
  plates 
  of 
  ArcJiceocidaris 
  

   were 
  imbricated 
  from 
  above 
  downwards, 
  and 
  the 
  ambulacral 
  from 
  

   below 
  upwards, 
  or 
  vice 
  versa, 
  must 
  remain 
  an 
  open'question 
  at 
  present, 
  

   as 
  we 
  are 
  not 
  in 
  possession 
  of 
  sufficiently 
  good 
  specimens 
  to 
  prove 
  

   tbis 
  point. 
  In 
  Ccdveria 
  the 
  peristome 
  is 
  covered 
  with 
  small 
  plates 
  §, 
  

   and 
  the 
  primary 
  tubercles, 
  as 
  in 
  ArcJiceocidaris, 
  are 
  hollow. 
  

  

  Archceocidaris 
  is 
  confined 
  to 
  rocks 
  of 
  Carboniferous 
  age. 
  

  

  Genus 
  Pal^chtnus 
  (Scouler), 
  M'Coy, 
  1844 
  1|. 
  PL 
  XXIV. 
  figs. 
  2 
  & 
  3. 
  

  

  The 
  plates 
  composing 
  the 
  test 
  of 
  this 
  genus 
  abut 
  end 
  to 
  end 
  in 
  the 
  

   regular 
  way, 
  and, 
  so 
  far 
  as 
  we 
  know, 
  there 
  is 
  no 
  overlapping 
  or 
  in- 
  

   termediate 
  membrane. 
  The 
  interambulacral 
  arese 
  are 
  composed 
  of 
  

   from 
  five 
  to 
  six 
  or 
  eight 
  rows 
  of 
  plates, 
  those 
  adjoining 
  the 
  ambulacra 
  

   pentagonal 
  as 
  in 
  ArcJiceocidaris, 
  the 
  others 
  hexagonal. 
  The 
  ambu- 
  

   lacra, 
  narrow, 
  and 
  sometimes 
  slightly 
  furrowed, 
  are 
  composed 
  of 
  two 
  

   alternating 
  rows 
  of 
  small 
  plates, 
  wider 
  than 
  long, 
  each 
  perforated 
  by 
  

   two 
  rows 
  of 
  pores. 
  The 
  apical 
  disk 
  consists 
  of 
  five 
  genital 
  plates 
  

   each 
  perforated 
  by 
  three 
  pores, 
  (ovarian 
  and 
  seminal), 
  the 
  fifth 
  

   being 
  somewhat 
  larger 
  (madreporiform 
  plate?), 
  and 
  five 
  ocular 
  

   plates, 
  each 
  perforated 
  by 
  two 
  openings 
  %. 
  

  

  Further, 
  in 
  the 
  apical 
  disk 
  there 
  are 
  eight 
  plates, 
  answering 
  to 
  the 
  

   suranal 
  of 
  the 
  Saleniadce** 
  , 
  

  

  It 
  becomes 
  apparent 
  that 
  Palcechinus 
  belongs 
  to 
  that 
  division 
  of 
  

   J;he 
  Perischoechinidce 
  which 
  more 
  nearly 
  approaches 
  the 
  normal 
  

   Echini, 
  from 
  which, 
  however, 
  it 
  differs 
  in 
  two 
  most 
  important 
  cha- 
  

  

  * 
  Synop. 
  Carb. 
  Foss. 
  Ireland, 
  1844, 
  p. 
  173. 
  t 
  J. 
  Young, 
  he. 
  cit. 
  p. 
  302. 
  

   X 
  Palaeontology 
  of 
  Illinois, 
  vol. 
  ii. 
  p. 
  294. 
  § 
  W. 
  Thomson, 
  I. 
  c. 
  p. 
  157. 
  

   || 
  L. 
  c. 
  p. 
  171. 
  1 
  Baily, 
  G-eol. 
  Mag. 
  vol. 
  ii. 
  p. 
  44. 
  

  

  ** 
  Baily, 
  two 
  papers 
  read 
  before 
  the 
  Eoyal 
  Geol. 
  Soc. 
  Ireland, 
  March 
  9th 
  and 
  

   April 
  12th, 
  1864, 
  " 
  On 
  the 
  Structure 
  of 
  Palcechinus." 
  [On 
  the 
  other 
  hand 
  Prof, 
  

   de 
  Koninck 
  has 
  described 
  and 
  figured 
  (Geol. 
  Mag. 
  vii. 
  p. 
  259, 
  pi. 
  7. 
  fig. 
  1) 
  the 
  

   apical 
  disk 
  of 
  Palcechinus 
  with 
  genital 
  plates 
  only, 
  four 
  perforated 
  by 
  three 
  

   pores, 
  and 
  the 
  fifth 
  by 
  one 
  pore.] 
  

  

  z 
  2 
  

  

  